FIG. 1 shows an example of the configuration of a conventional WDM (Wavelength Division Multiplexing) transmission system. In FIG. 1, an optical transmitter 50 is composed of semiconductor lasers (for example, distribution feedback lasers: DFB-LD) 51-1 to 51-n having different wavelengths defined in a transmission specification (for example, the ITU-T G.692 recommendation), optical modulators 52-1 to 52-n for modulating optical outputs from the semiconductor lasers by means of transmitted signals, a multiplexer 53 for multiplexing modulated signal lights to output a WDM signal light, and an optical amplifier 55. An optical receiver 70 connected to the optical transmitter 50 via a transmission path optical fiber 60 is composed of an optical amplifier 71 for amplifying the transmitted WDM signal light, a demultiplexer 72 for demultiplexing the WDM signal light into signal lights of different wavelengths, and receivers 73-1 to 73-n for receiving the signal lights of the different wavelengths.
The semiconductor lasers require a wavelength stabilizing circuit to maintain the wavelength accuracy defined in the transmission specification because they are characterized by having their oscillation wavelengths shifted due to deviations in temperature and injected current and varied with temporal deviations. Since the wavelength stabilization must be carried out for each semiconductor laser, the area of the apparatus occupied by the wavelength stabilizing circuit increases consistently with the number of wavelength multiplexing operations required and with the wavelength multiplexing density. Accordingly, the costs of a light source used must be reduced in order to realize dense WDM transmissions involving a large number of wavelengths.
Such a configuration with a plurality of semiconductor lasers employs a method of generating a multi-wavelength light composed of a plurality of wavelengths, by using a demultiplexer with a plurality of output ports to filter (spectrum slicing) a continuous or discrete optical spectrum of a wide band output from a single optical element or circuit. Light sources for generating such a continuous optical spectrum of a wide band include optical amplifiers for outputting an amplified spontaneous emission (ASE) light. Light sources for generating discrete optical spectra include pulsed light sources for outputting a recurrent short optical pulse, and optical circuits for generating a sideband composed of discrete modes by modulating (intensity or phase modulation) a CW (continuous wave) light output from a semiconductor laser.
A light obtained by slicing a spectrum of the ASE light, however, is incoherent and thus unsuitable for dense WDM transmissions involving a large number of wavelengths. On the other hand, a repetition short optical pulse or a discrete spectrum obtained by modulating a continuum has longitudinal modes discretely distributed on a frequency axis at the same intervals as a repetition frequency; these longitudinal modes are very coherent. Thus, this optical circuit can be replaced for the conventional system and is suited for the wavelength-dividing multiplexing method. In general, however, the above described multi-wavelength light obtained by slicing an optical spectrum of a wide band has large power level deviations among channels, thus requiring such power adjustments that the wavelength channels have an equal power.
Another method comprises eliminating power level deviations by using an optical filter with a transmission characteristic reverse to that of an optical spectrum of a multi-wavelength light in order to restrain the power level deviations. For the recurrent short optical pulse, a method is used which comprises generating a flattened wide-band optical spectrum of a wide band by positively using a non-linear optical fiber, as in a process of generating a supercontinuum by allowing a light to pass through the non-linear optical fiber.
While flattening involved in such supercontinuum generation, the input power of a given seed pulse, the dispersion profile of the non-linear fiber, and the fiber length ought to be designed so that an output optical spectrum of the seed pulse is flattened and has a wide band after being output from the non-linear fiber. Such design and production, however, is in effect difficult. Further, since the shape of the optical spectrum is uniquely determined by its design parameters, it is impossible to dynamically control power level deviations among the longitudinal modes. Moreover, the process of flattening an optical spectrum using the optical filter with the reverse transmission characteristic also has problems in design difficulty and uniquely decided output spectrum as in the above described supercontinuum generation.
It is a first object of the present invention to provide an optical-spectrum flattening method and apparatus which has a simple and inexpensive configuration and which enables the control of power level deviations among the modes of a discrete optical spectrum.
It is a second object of the present invention to provide a collective multi-wavelength generating apparatus which has a simple and inexpensive configuration and which makes it possible to generate, without the need to design a complicated optical circuit, a WDM signal with a flattened optical spectrum by modulating a light with a single central frequency by means of an electric signal of a particular pulse repetition frequency.
It is a third object of the present invention to provide a coherent multi-wavelength signal generating apparatus, in a configuration controllable shape of an optical spectrum of a multi-wavelength light, which controls the shape of the optical spectrum of the multi-wavelength light such that a predetermined RIN (Relative Intensity Noise) or SNR (Signal to Noise Ratio) value required from parameters of transmission system, the type and distance of optical fibers, the number of repeaters is obtained, by using the above described multi-wavelength generating apparatus to generate the multi-wavelength light.
It is a fourth object of the present invention to provide a multi-wavelength light source having a simple and inexpensive configuration that does not require a complicated optical circuit to be designed, the light source being realized by taking a plurality of lights into the above described multi-wavelength generating apparatus and making it possible to generate a WDM signal with a flattened optical spectrum without any interfering noise.